To assist driving, a view of a road condition ahead of a vehicle is imaged by an imaging device mounted in the vehicle, and a lane mark is detected by subjecting the captured image to image processing. There has been known conventional image processing for detecting a lane mark, which uses Hough transform.
Patent Document 1 discloses a road configuration estimating device adapted to detect a lane mark (lane indication line) from a captured image ahead of a vehicle by using a method other than the Hough transform. The road configuration estimating device sets a plurality of trapezoidal windows in the captured image such that the lane indication line is included therein, and detects edge points in the captured image by edge extraction processing. Then, the road configuration estimating device checks the number of the edge points on all segments, each of which has one point on the upper base and one point on the lower base of each window (two points facing each other in the direction of the lane) as both ends thereof, and detects the segment having the greatest number of edge points as the detection straight line of the window. Further, the road configuration estimating device determines whether the window indicates a vehicle from an adjacent lane entering into the traveling lane of a self-vehicle by determining whether the number of the edge points of the detection straight line of the window is a threshold value or more, and focuses on a window showing no entry of the vehicle, thus detecting a lane indication line of the entire captured image.